assume cs:code

a segment
    db 1, 2, 3, 4, 5, 6, 7, 8
a ends

b segment
    db 1, 2, 3, 4, 5, 6, 7, 8
b ends

d segment
    db 0, 0, 0, 0, 0, 0, 0, 0
d ends

code segment
    start:
        mov bx, 00h
        mov cx, 08h

        s:
            mov dx, 00h

            mov ax, a
            mov ds, ax
            add dl, [bx]

            mov ax, b
            mov ds, ax
            add dl, [bx]

            mov ax, d
            mov ds, ax
            mov [bx], dl

            inc bx

            loop s

        mov ax, 4c00h
        int 21h
code ends

end start